WIP... don't use this version, there's an error on KnownUri. I was trying to get the relationships selectable on the edit page, but having form problems. Will fix in two days.
WIP - still not getting the relationship-saving correct in the controller (or perhaps in the form... or maybe the model's definition. Unclear.)
Wow, that was much harder than it should have been. Smells like another case where having subclasses would have been handy (for each of the known uri relationship types), but I still haven't found a satisfying solution to that problem in Rails.